The present invention is related to an electrolytic capacitor. More specifically the present invention is related to an electrolytic capacitor comprising intrinsically conductive polymeric cathode layers capable of achieving a high break down voltage (BDV) wherein the BDV exceeds the dielectric formation voltage which was previously not considered feasible with polymeric cathode layers.
Solid electrolytic capacitors with intrinsically conductive polymers as the cathode material have been widely used in the electronics industry due to their advantageous low equivalent series resistance (ESR) and “non-burning/non-ignition” failure mode. Intrinsically conductive polymer, more commonly known as conductive polymer, is electrically conductive in the molecular level. In other words, a single molecule (a polymer chain) of this type of polymer is conductive, which distinguishes itself from other groups of polymeric materials whose electrical conductivity is due to percolation between conductive particles within the polymer. The example of the latter is non-conductive polyester filled with conductive carbon back particles. The intrinsically conducting polymer can exist in many physical forms including solid, solution, and liquid dispersion.
The backbone of a conductive polymer consists of a conjugated bonding structure. The polymer can exist in two general states, an undoped, non-conductive state, and a doped, conductive state. In the doped state, the polymer is conductive but of poor processibility due to a high degree of conjugation along the polymer chain, while in its undoped form, the same polymer loses its conductivity but can be processed more easily because it is more soluble. When doped, the polymer incorporates anionic moieties as constituents on its positively charged backbone. In order to achieve high conductivity, the conductive polymers used in the capacitor must be in doped form after the completion of processing, although during the process, the polymer can be undoped/doped to achieve certain process advantages.
Various types of conductive polymers including polypyrrole, polyaniline, and polythiophene are described for use in Ta capacitors. The major drawback of conductive polymer capacitors regardless of the types of conductive polymers employed, is their relatively high ratio between formation voltage of the dielectric and rated voltage of the capacitor, which affected charge efficiency of these capacitors. Furthermore, upon failure capacitors with a conductive polymer cathode fail in a closed circuit mode, wherein the capacitor essentially represents an electrical short, which can be detrimental to the circuit.
There has been a long-standing desire in the art to provide a capacitor comprising a conducting polymeric cathode suitable for use at lower ratio between formation and rated voltages, which requires breakdown voltage exceeding formation voltage. It was also a long-standing desire in the art to provide tantalum capacitor with open failure mode. Through diligent research the present inventors have achieved what was previously not considered feasible.